You got it all wrong... adults fall down too. If baby's saw everything inverted, they'd fall up

I always explained pinholes with the fact that light travels in straight lines/rays through air (may be a half-truth, but good enough for most people and easy to prove) and a little sketch of a camera. It's not hard to understand that only the rays of light that form the image are let through and all others are blocked. When it comes to more complicated cameras, "magic" is the more useful explanation, as I could explain it all, but would take days.